Social Enterprise Alliance - "Social Enterprise (n.) Any earned-income business or strategy undertaken by a nonprofit to generate revenue in support of its charitable mission.
REDF - "...social entrepreneurs ... involved in the operation of social-purposes enterprises - market-based businesses that pursue a social mission." Includes a list of social enterprises by state.
New Economics Foundation - "Social enterprises are organisations that achieve social or environmental aims primarily through trading." (British site)
Enterprising Nonprofits : a toolkit for social entrepreneurs by Gregory Dees. ISBN: 0471397350
Social Entrepreneurship: The Art of Mission-Based Venture Development by Peter C. Brinckerhoff. ISBN: 0471362824
Generating and Sustaining Nonprofit Earned Income: A guide to successful enterprise strategies by Sharon Oster. ISBN: 078797238X
Starting and Managing a Nonprofit Organization: A legal guide by Bruce Hopkins - ISBN: 047139727X
Effective economic decision-making by nonprofit organizations / Dennis R. Young, editor ; sponsored by the National Center on Nonprofit Enterprise and The Foundation Center. ISBN: 1931923698
Community Wealth Ventures, Inc. - directory of nonprofits with earned-income projects, reports on social enterprise in the U.S., venture philanthropy and more.